The invention relates to a method and an apparatus for determination of the rotor position of an electric motor, in particular of a brushless electric motor. A method for controlling an electric motor, in which the position of a rotor relative to a stator is determined, is known, for example, from DE 44 37 793 C2. Analogously to the terminology used in this document, the terms “rotor” and “stator” in the following text refer to the functional basic elements of an electric motor which assume a variable position (rotor) or a fixed position (stator) relative a fixed coordinate system. This therefore also covers linear motors as electric motors.
In the method which is known from DE 44 37 793 C2, the relationship between a known signal which is applied to the electric motor and the movement of the rotor stimulated by this is used to determine the position of the rotor, in which case, by way of example, the change in the position of the rotor is measured by means of an incremental, optical measurement system. There is therefore no need to directly measure the rotor position by means of an absolute measurement system. However, depending inter alia on the motor geometry, not only an absolute measurement system but also an incremental measurement system can represent considerable hardware complexity. This applies in particular to hollow-shaft motors, as are used by way of example in machines which process plastic. On the other hand, in the case of the method according to DE 44 37 793 C2, the magnitude of the amplitude of the movement which is excited by the known signal that is applied often represents a disadvantage.